Significant advances in the treatment of myeloma have resulted in a high rate of complete remissions;however, all patients eventually relapse and succumb to the disease. The bone marrow microenvironment, particularly the cells involved in bone formation and destruction, are intimately involved in the disease process as regulators of myeloma growth and tumor manifestations. The overall goal of our research is to develop a new paradigm of myeloma therapy, whereby control of bone disease helps to control myeloma development and progression. Working toward this goal, we propose experiments to elucidate the role of osteoblasts, the bone-forming marrow cells, in the disease process and to develop approaches to control myeloma by controlling the associated lytic bone disease. Our experiments will address two specific aims to investigate in vitro and in vivo the consequences of interactions between myeloma cells and osteoblasts, at both the physiological and molecular levels. We will examine in vivo in the SCID-hu model whether increasing the number and activity of osteoblasts results in bone formation and impacts myeloma growth (Specific Aim 1). These experiments address our hypothesis that increasing osteoblast numbers and activity will increase bone formation and will control growth and survival of myeloma cells. We will attempt to increase osteoblast number and activity by injecting osteoblast progenitor cells and parathyroid hormone, individually and in combination. We will also elucidate in vitro the nature and consequences of interactions between myeloma cells and bone marrow-derived MSC on osteoblast differentiation and survival and on myeloma cell survival (Specific Aim 2). These experiments are designed to pursue our hypothesis that, in patients with lytic bone disease, myeloma cells disrupt the mesenchymal stem cell to osteoblast differentiation process, and the resultant elimination of osteoblasts facilitates myeloma cell survival and disease progression. Preliminary results suggest that the effects of intercellular interactions will be heterogeneous, and we propose experiments to examine potential sources for this variety. We will employ state-of-the-art proteomics technologies to further investigate these interactions at the molecular level. Our newly developed in vivo and in vitro models of myeloma cell/osteoblast interactions and myeloma disease progression, combined with advanced proteomics technologies, are powerful tools for deciphering critical aspects of myeloma biology, identifying targets for effective therapeutic interventions, and developing molecular tools for evaluating treatment efficacy. The results of the proposed study will lead us to the next stage in which we will design treatment protocols aimed at improving myeloma-related bone disease and test treatment efficacy in preventing myeloma relapses and disease progression.